Discovery of the hot companion to the two-day cepheid HD 129708.

Abstract

The Cepheid HD 129708 has been observed with the International Ultraviolet Explorer satellite, and a hot companion star (B4-5) has been discovered. Comparison of the observed energy distribution with theoretical atmosphere models gives T_e = 16,000±1000K and log g = 4.0±0.5 for the hot companion. A visual magnitude difference ΔV = 2^m.4±0^m.2 between the hot secondary and the Cepheid is calculated, which implies M_v ≡ -3^m.0±0^m.3 for the Cepheid. This number is to be compared with M_v = -2^m.8, predicted from the ridge line of the period-luminosity relationship, indicating that the Cepheid is pulsating in the fundamental mode.