Spectroscopic Discovery of Brown Dwarf–like Secondary Stars in the Cataclysmic Variables LL Andromedae and EF Eridani

Abstract

Infrared spectroscopic observations of LL Andromedae and EF Eridani are presented. Our K-band spectrum of LL And reveals the presence of methane absorption in the secondary star indicating an effective temperature 1300 K, similar to a T-type methane brown dwarf. The secondary star in EF Eri is seen to be warmer with an effective temperature of ~1650 K and a spectroscopic classification consistent with an L4-5 star. Both secondaries have theoretical mass estimates of 0.04-0.055 M☉ (40-55 MJup). Our IR spectroscopic observations of LL And and EF Eri provide the first direct proof of the theoretical prediction that post-period minimum cataclysmic variables contain brown dwarf-like secondary stars.