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In this paper we present the first phase-resolved, high-resolution, UV spectroscopy of V Sge. HST GHRS observations were obtained covering 1.5 orbital cycles of V Sge in a low state. We covered the Heii 1640 line and the Civ 1550 doublet alternately, and also obtained one spectrum covering Mgii 2800.

We create continuum and line flux light curves for the Heii and Civ spectra. A secular decrease in continuum and line strength occurs throughout our observation. The continuum light curves show a deep primary eclipse but no obvious secondary eclipse. The line fluxes show no significant changes during primary or secondary eclipses.

The Heii line shows dramatic orbital variations in shape, but it is always purely an emission line, i.e., it shows no sign of a P Cygni profile. The Civ doublet shows a P Cygni profile in which the absorption vanishes at primary mid-eclipse. This locates the source of the P Cygni line in the primary star wind. The maximum velocity of absorption in the P Cygni profile is 700km.s^–1, much smaller than the HWZI of the Civ emission which is 1500km.s^–1. We attribute this to the presence of a broad Civ emission component underlying the primary star P Cygni profile. The Civ line also varies with orbital phase. In particular, the velocity at the blue edge of the absorption varies between -700 and -250km.s^–1 as a function of orbital phase. The red and blue peaks of the profile also vary around the orbit, with the red peak at maximum strength near phases 0.5-0.7. The blue peak is at maximum strength near phases 0.1-0.2.

We can explain the detailed behaviour of the Heii and Civ line profiles in the context of a colliding wind model. Here the winds from each star collide, forming a shock which, due to the rapid rotation of the binary, is wrapped around both stars. The Civ P Cygni line comes from the primary star wind, which has a terminal velocity of ∼700km.s^–1. We attribute the Heii line and the broad Civ emission to a fast, optically thin wind from the secondary star. This wind has a terminal velocity of ∼1500km.s^–1.

Interstellar lines due to Ci, Ci*, Civ, Mgii and Pii are detected. Column densities are measured for Ci and Pii. The Ci column density is used to estimate the hydrogen column density. We find that n_H is of order 10¹⁹atom.cm^–2.