Dear colleagues, We are preparing to trigger a coordinated HST/RXTE target of opportunity programme to observe XTE J1550-564, mainly in the UV. We therefore urgently require up-to-date magnitudes of the optical counterpart (Orosz et al. IAUC 7009, http://vsnet.astro.psu.edu/users/orosz/xte.html) and B-V and/or V-R colours. These will give us an idea of what UV brightness to expect. We would be grateful for any information that can be provided to rih@star.cpes.susx.ac.uk and chaswell@star.cpes.susx.ac.uk. C. Haswell, R. Hynes on behalf of the HST/RXTE collaboration