FIRST J102347.6+003841: The first radio-selected cataclysmic variable

We have identified the 1.4 GHz radio source FIRST J102347.6+003841 (hereafter FIRST J1023+0038) with a previously unknown 17th magnitude Galactic cataclysmic variable (CV). The optical spectrum resembles that of a magnetic (AM Herculis or DQ Herculis type) CV. Five nights of optical CCD photometry showed variations on timescales of minutes to hours, along with rapid flickering. A reexamination of the FIRST radio-survey data reveals that the radio detection was based on a single 6.6 mJy flare; on two other occasions, the source was below the 1 mJy survey limit. Several other magnetic CVs are known to be variable radio sources, suggesting that FIRST J1023+0038 is a new member of this class (and the first CV to be discovered on the basis of radio emission). However, FIRST J1023+0038 is several optical magnitudes fainter than the other radio-detected magnetic CVs. It remains unclear whether the source simply had a very rare and extraordinarily intense radio flare at the time of the FIRST observation, or is really an unusually radio-luminous CV; thus, further observations are urged.