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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 (1977) 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 ~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 May 2004 and June 2010. Variability was analyzed 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 dataset 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 mags 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-half-maximum FWHM=27000+-280 km s/s. If the system follows the FWHM -- K_2 correlation found by Casares (2015), this would correspond to a velocity semi-amplitude of the donor star of K_2=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_solar and <16 M_solar respectively.