1 Introduction

Recent observations of intervening O VI absorbers in HST-STIS Echelle spectra of bright, low redshift QSOs have provided strong evidence that in the local universe a considerable fraction of baryonic matter might be "hidden'' in a warm ($\sim$$10^{5}\,$K) intergalactic medium (Savage et al. 1998; Tripp et al. 2000; Tripp & Savage 2000). This observation is in accordance with models of hierarchical structure formation by Cen & Ostriker (1999) and Davé et al. (2001) which predict that a considerable fraction of all baryons reside in a warm-hot phase of the intergalactic medium (WHIM) shock-heated to temperatures of $10^{5}{-}10^{7}\,$K. The same models predict that the fraction of baryons residing in this WHIM increases strongly with decreasing redshift from less than 5% at z = 3 to 30-40% at z = 0. Can this prediction be verified or disproved by observations, or can observations even impose constraints on the models? This appears difficult for various reasons. First of all, the WHIM is difficult to detect (cf. Davé et al. 2001), both as diffuse X-ray emission of the hotter parts or in absorption through the O VI doublet. In addition, the temperature distribution of the WHIM varies with redshift so that a complete census would require the detection of all components as a function of redshift. The warm O VI component has the additional complication that both the oxygen abundance and the ionization process cannot be determined from O VI observations alone. While at low redshift (z < 0.3) collisional ionization is the most probable process since the ionizing extragalactic UV background is diluted, O VI can be produced easily by photoionization at redshifts $\geq 2$ and has been observed to be ubiquitous in the low-density IGM (Schaye et al. 2000). On the other hand O VI is not expected to be produced by photoionization for $z \geq 3$ since the reionization of He II is incomplete (Reimers et al. 1997; Heap et al. 2000) and the IGM therefore opaque to photons with energies above 4 Rydberg. The intermediate redshift range remains which for z < 1.9 requires high-resolution UV-spectroscopy of a bright, high-redshift QSO. In this paper, we present combined high-resolution HST/STIS observations of O VI absorption supplemented by ESO-VLT/UVES spectroscopy of the accompanying H I and C IV lines in the brightest known intermediate redshift QSO HE 0515-4414 ( $z_{\rm em} = 1.73$, B = 15.0) discovered by the Hamburg/ESO Survey (Reimers et al. 1998). The data have been taken mainly with the aim of studing the evolution of the Ly $\alpha$ forest and its metal content in the range z = 1 to 1.7. In this first paper we concentrate on the intervening O VI absorption.