Irradiation of the secondary star in X - ray Nova Scorpii 1994 ( = GRO J 1655 – 40 )

We have obtained intermediate resolution optical spectra of the black-hole candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical active state (R ∼15.05). We measure the radial velocity curve of the secondary star and obtain a semi-amplitude of 279±10km s−1; a value which is 30 per cent larger than the value obtained when the source is in quiescence. Our large value for K2 is consistent with 60 −7 per cent of the secondary star’s surface being heated; compared to 35 per cent, which is what one would expect if only the inner face of the secondary star were irradiated. Effects such as irradiation-induced flows on the secondary star may be important in explaining the observed large value for K2.