Revisiting the post-common-envelope binaries HZ 9, KV Vel and Abell 65

ABSTRACT New time-resolved photometric observations of three close but detached white dwarf/red dwarf binaries have enabled an in-depth analysis of their properties, combining new data with known information. For HZ 9, time-resolved photometric data have not been published previously. Our data reveal sinusoidal variations, with the orbital period showing unusual phase shifts between expected and observed light maximum epochs when combining our photometry with the precise radial velocity ephemeris. There are also variations of the photometric amplitude between 0.04 and 0.11 mag. Phase shifts and amplitude variations could be the result of a relatively strong magnetic activity of the secondary M star component, which seems to superimpose the usual reflection effect. This effect is dominant in the two remaining targets of our study. We improve the accuracy of the photometric ephemeris of KV Vel, complementing published data with light curves from the All-Sky Automated Survey (ASAS) and ASAS-SN and with our own observations. Finally, we also add new data for the central star of the planetary nebula Abell 65, complementing and confirming its previously published photometric ephemeris.

The UBV Color Evolution of Classical and Symbiotic Novae

We identified a general course of classical nova outbursts in the <em>B − V</em> vs. <em>U − B</em> diagram. It has been reported that novae show spectra similar to A–F supergiants near optical light maximum. However, they do not follow the supergiant sequence in the color-color diagram, neither the blackbody nor the main-sequence sequence. Instead, we found that novae evolve along a new sequence in the pre-maximum and near-maximum phases, which we call the nova-giant sequence. This sequence is parallel to but Δ<em>(U − B)</em> ≈ −0.2 mag bluer than the supergiant sequence. After optical maximum, its color quickly evolves back blueward along the same nova-giant sequence and reaches the point of free-free emission (<em>B − V</em> = −0.03, <em>U − B</em> = −0.97) and stays there for a while, which is coincident with the intersection of the blackbody sequence and the nova-giant sequence. Then the color evolves leftward (blueward in <em>B − V</em> but almost constant in <em>U − B</em>) due mainly to development of strong emission lines. This is the general course of nova outbursts in the color-color diagram, which is deduced from eight well-observed novae including various speed classes. For a nova with unknown extinction, we can determine a reliable value of the color excess by matching the observed track of the target nova with this general course. This is a new and convenient method for obtaining color excesses of classical novae. Using this method, we redetermined the color excesses of nineteen well-observed novae.

A Theoretical Period-Luminosity Relation of Dwarf Cepheids

Dwarf Cepheids (large-amplitude δ Sct variables and SXPhe variables in the GCVS) are late A-type stars near light maximum and early-F stars near light minimum. In the lower part of the instability strip where these stars are found the pulsation constant Q varies only slightly. As pointed out by Andreasen, Hejlesen, and Petersen (1983) this makes it easy to transform a theoretical HR or alternatively a (logTe, logg) diagram to a theoretical logTe, logP0) diagram by the use of the relation. This follows from the equations:where P0 is the fundamental period of pulsation. The periods, of course, can be determined with very high accuracy.

β Cas: Simultaneous Stromgren Photometry

The observations were carried out in August, 1983 using the 75 cm telescope at Sierra Nevada Observatory in Spain. This telescope is equipped with a six-channel uvbyβ photometer for simultaneous measurements in uvby and the narrow and wide Hβ channels, respectively. The observations were collected during four nights in the four uvby filters and one more night was devoted to measuring β Cas in the n and w bands of the Hβ Crawford system. In all cases, a calibrated neutral filter was used for β Cas. The comparison star was Cl=HR 9085 (V=5.m55, F0III).The analysis of frequencies was carried out using the Discrete Fourier Transform method, as described in López de Coca et al. (1984), to the observational uvby points collected. The periodograms showed a principal peak at v=9.91 cd−l. After prewhitening for this frequency, the resulting periodograms did not show any trace of another peak, suggesting the monoperiodic nature of this star, in agreement with Antonello et al. (1986). Under the assumption of monoperiodicity of β Cas, the classical O-C method can easily be applied. Nine times of maxima were obtained for the four nights by using the method described in Rodriguez et al. (1990), where each light maximum has been derived as an average over the four uvby bands. When the O-C method was applied, the least squares fit converged to a linear ephemeris with the following elements: To=2445568.d5097 (±0.d0007) and Po=0.d10091 (±0.d00002). This linear ephemeris satisfactorily reproduces our data of β Cas.

Mass of the Ejected Envelope of LV Vulpeculae

LV Vulpeculae is a fast nova with a relatively smooth light curve (Raikova, 1981). On Fig. 1 the picture of the radial velocities, displayed during the outburst is shown. It is based on our measurements and the published ones (a more detailed paper will appear in C. R. Acad. Bulg. Sci.). The premaximum absorption line system (A) exhibits a rapidly decreasing velocity. It has been observed for the last time on April 17.52 (Hutchings, 1970) together with the diffuse enhanced system (B) of velocity −1260 km/s. Since the presence of the last system is an indication that the light maximum has passed, we can limit a 2h40m interval in which it had occured (on April 17.385 the brightness is still up). We assume that the light maximum has taken place at the middle – on April 17.45. On April 18 the diffuse enhanced line system, now of −1400 km/s, and the principal one (C) of −780 km/s exist. It is worth-while to emphasize that in LV Vul spectrum the diffuse enhanced system appeared before the principal one.

Light and Temperature Govern Germination and Storage of Caladium Seed

Caladium hortulanum Birdsey cv. Candidum seed failed to germinate without light; maximum germination required daily, incandescent light of ≤4 hours. Lengthening daily lighting periods progressively reduced the days to 50% relative germination (T50) from 20 to 8, and days between 10% and 90% relative germination (T90 – T10) from 16 to 5. T50 and T90 – T10 were shortest (≈ 8 days) at 25 and 30C, while total or absolute germination percentage (G) was highest at ≈ 90%. G was 94% for seeds harvested immediately, but 75% or 38% for seeds that remained in fruits for 3 or 12 weeks after fruit abscission from the spadix. Total absolute germination was reduced from 95% to 87% when seed moisture contents declined to <14%. Seed storage for 7 days at from 10 to – 80 C-caused no reduction in G. Seeds were stored 6 months at 15C and 22%, 33%, or 52% RH without change in G, but storage at 5 or 25C and 11%, 75%, or 95% RH significantly reduced germination.

A non-eclipsing binary model of the symbiotic star AG Dra

The light variation of AG Dra is considered as a result of a variation of free-free emission from a gaseous envelope around the hot component. The emission measure of the envelope at a light maximum is estimated to be about Ne2V = 6.5 × 1059 cm−3. The amplitude of the light variation in the U band should be less than 1.4 mag. which is consistent with the observed results. The emission measure of the envelope might vary according to a variation of mass transfer rate in an elliptical binary system.

Polarimetric Observations of Hydrogen Deficient Stars

Polarimetric observations of HD 30353, SU Tau, XX Cam, R Cr B,UV Cas, BD+13 3224,BD+10 2179 and HD 124448 are presented. The linear polarization of HD 30353 is found to vary appreciably at Hα over time scale as short as one day. It is also found that SU Tau shows significant variation in polarisation even at light maximum.