We summarize the development of the field of Double Periodic Variables (DPVs,
Mennickent et al. 2003) during the last fourteen years, placing these
objects in the context of intermediate-mass close interacting binaries
similar to ? Persei (Algol) and ? Lyrae (Sheliak) which are generally called
Algols. DPVs show enigmatic long photometric cycles lasting on average about
33 times the orbital period, and have physical properties resembling, in
some aspects, ? Lyrae. About 200 of these objects have been found in the
Galaxy and the Magellanic Clouds. Light curve models and orbitally resolved
spectroscopy indicate that DPVs are semi-detached interacting binaries
consisting of a near main-sequence B-type star accreting matter from a
cooler giant and surrounded by an optically thick disc. This disc
contributes a significant fraction of the system luminosity and its
luminosity is larger than expected from the phenomenon of mass accretion
alone. In some systems, an optically thin disc component is observed in well
developed Balmer emission lines. The optically thick disc shows bright zones
up to tens percent hotter than the disc, probably indicating shocks
resulting from the gas and disc stream dynamics. We conjecture that a
hotspot wind might be one of the channels for a mild systemic mass loss,
since evidence for jets, winds or general mass loss has been found in ?
Lyrae, AUMon, HD170582, OGLE05155332-6925581 and V393 Sco. Also, theoretical
work by Van Rensbergen et al. (2008) and Deschamps et al. (2013) suggests
that hotspot could drive mass loss from Algols. We give special
consideration to the recently published hypothesis for the long-cycle,
consisting of variable mass transfer driven by a magnetic dynamo (Schleicher
and Mennickent 2017). The Applegate (1992) mechanism should modify
cyclically the equatorial radius of the chromospherically active donor
producing cycles of enhanced mass loss through the inner Lagrangian point.
Chromospheric emission in V393 Sco, an optically thicker hotspot in the
high-state of HD170582 and evidence for magnetic fields in many Algols are
observational facts supporting this picture. One of the open questions for
this scenario is why, among the Algols showing evidence for magnetic fields,
the DPV long-cycle is present only under some combinations of stellar
parameters, particularly those including the B-type gainers. Other open
questions are what are the descendants of these interesting binaries, how
much mass contain the discs around the likely rapidly rotating gainers, and
the role played by the outflows through the Lagrangian L2 and L3 points
reported in a couple of systems.
Massive Black Holes Regulated by Luminous Blue Variable Mass Loss and Magnetic Fields
