DX Andromedae

DX Andromedae (often abbreviated to DX And) is a cataclysmic variable star in the constellation Andromeda. It has a typical apparent visual magnitude of 15.5 during the quiescent phase, but becomes brighter (up to a magnitude of 11.0) during outbursts recurring with a mean cycle length of 330 days, thus is classified as a dwarf nova of the SS Cygni type.

System
The system of DX Andromedae consists of a white dwarf accreting matter from a donor star through an accretion disk. The system shows a spectral type at quiescence of K1 V (plus emission lines), but the donor is considered to be a hydrogen-deficient star that has evolved off the main sequence and is overfilling its roche lobe. The donor star dominates the visible spectrum during the quiescent phase.

The spectral classification of DX Andromedae as a whole is classified as peculiar of the U Geminorum type. The spectrum of the donor star is itself peculiar; enhanced absorption lines of CaI and CrI have been observed.

Variability
The outburst of dwarf novae are thought to arise from a thermal instability of the disk, that increases mass flow and temperature and hence the luminosity increases. In DX Andromedae different outbursts don't reach the same peak luminosity and don't occur at a regular rate, but they retain the same shape in the decaying part. It seems that the peak luminosity is correlated to the length of the preceding cycle, and the more luminous is the outburst, the faster the luminosity peak is reached. These trends can be explained by matter accumulating in the disk; more matter is accumulated in the disk during a longer cycle, and it can power a stronger outburst.