Institute for Astronomy and Astrophysics

Binaries composed of main-sequence star and white dwarf

Binaries composed of a white dwarf and a main-sequence star are a frequent outcome of binary evolution. If the separation between the two components is small matter flows from the main-sequence star onto the white dwarf. Such accreting systems are called cataclysmic variables (CVs). Very little is known about CVs in which the donor is a very low-mass star or a brown dwarf (spectral type L), i.e. it belongs to the class of ultracool dwarfs (see topic "Magnetic activity"). We work on the identification and characterisation of such systems through optical and X-ray observations.

Fig.: X-ray brightness variation of the accreting binary SDSS J121209.31+013627.7 obtained with the ESA satellite XMM-Newton. Mass flows from an L dwarf onto a white dwarf and produces a so-called accretion spot on the white dwarf surface. The periodic variation results from the orbital motion in the binary system in the course of which the X-ray bright accretion spot rotates in and out of the line-of-sight. (B.Stelzer)