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Astron. Astrophys. 324, L1-L4 (1997)

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1. Introduction

Gravitationally lensed quasars with multiple images provide a unique tool for determining the cosmological parameters independently of the classical methods. In particular, time delays between the images of a lensed quasar can be used to determine the Hubble constant H0 (e.g. Refsdal 1964, Grogin & Narayan 1996).

Extensive photometric monitoring of lensed quasars has been undertaken to obtain accurate light curves for the most promising objects. Unfortunately, precise time delays have been difficult to obtain due to poor temporal sampling of the light curves, possible contamination by microlensing and short time scale events.

Light curves have recently been obtained for the quadruply imaged quasar PG 1115+080 (Schechter et al. 1997). The good time coverage, together with the fact that microlensing does not seem to significantly contaminate the intrinsic photometric variations of the lensed source, yield measurements of two time delays between three components of the system (Schechter et al. 1997, Bar-Kana 1997). Keeton and Kochanek (1997, hereafter KK97) showed that it was difficult to constrain H0 because of degeneracies in the lens models. However, KK97 also showed that the constraints could be improved by measuring the position of the lensing galaxy relative to the quasar images better than was possible with the pre-refurbishment HST observations of Kristian et al. (1993, hereafter K93). The aim of the present paper is to provide a better determination of the galaxy position and, thus, an improved estimate of H0.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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