The Historical X-ray Transient KY TrA in quiescence

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 alpha = 15:28:16.97 and delta = -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 similar to 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 similar to 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 H alpha emission characteristic of X-ray transients in quiescence with a full width at half-maximum FWHM = 2700 +/- 280 km s(-1). If the system follows the FWHM-K-2 correlation found by Casares, this would correspond to a velocity semi-amplitude of the donor star of K-2 = 630 +/- 74 km s(-1). 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 approximate to 9 M-circle dot and <16 M-circle dot, respectively.