2015MNRAS.454.3351Z

We present deep optical images of the historical X-ray Transient KY TrA in quiescence from which we confirm the identification of the counterpart reported by Murdin et al. and derive an improved position of α = 15:28:16.97 and δ = -61:52:57.8. In 2007 June we obtained I, R and V images, where the counterpart seems to be double indicating the presence of an interloper at ∼ 1.4 arcsec NW. After separating the contribution of KY TrA we calculate I = 21.47±0.09, R = 22.3±0.1 and V = 23.6±0.1. Similar brightness in the I-band was measured in 2004 May and 2010 June. Variability was analysed from series of images taken in 2004, spanning 0.6 h, and in two blocks of 6 h during 2007. We find that the target is not variable in any data set above the error levels ∼ 0.07 mags. The presence of the interloper might explain the non-detection of the classic ellipsoidal modulation; our data indicates that it contributes around half of the total flux, which would make a variation <0.15 mag not detectable. A single spectrum obtained in 2004 May shows the Hα emission characteristic of X-ray transients in quiescence with a full width at half-maximum FWHM = 2700±280 km/s. If the system follows the FWHM-K2 correlation found by Casares, this would correspond to a velocity semi-amplitude of the donor star of K2 = 630±74 km/s. Based on the outburst amplitude and colours of the optical counterpart in quiescence we derive a crude estimate of the orbital period of 8 h and an upper limit of 15 h which would lead to mass function estimates of ~9 M_☉ and <16 M_☉, respectively.