X-Shooting EF Eridani: further evidence for a massive white dwarf and a sub-stellar secondary

Abstract

High resolution spectral observations of the polar EF Eridani obtained in a low state with X-Shooter revealed narrow emission lines from the irradiated secondary. The lines were most prominent in the near-IR Ca2-triplet, the more intensive Halpha line had additional emission likely originating from an accretion stream. The lines with a radial velocity amplitude, K2' = 385 +- 4 kmps, serve as tracer of the otherwise unseen companion. The mass function implies a massive white dwarf with Mwd > 0.65 Msun at 3sigma confidence, and a short distance to the binary, d ~ 111 pc (<145pc at 3sigma confidence). The spectral energy distribution from the UV to the IR together with the high mass ratio gives further strong evidence of EF Eri being a post period-minimum object with M2 < 0.06 Msun