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Astron. Astrophys. 339, L73-L76 (1998)

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1. Introduction

HE 1104-1805 (the `Double Hamburger', [FORMULA]) is one of the brightest multiply imaged QSOs in the sky: Around discovery in 1993, its B band magnitudes were 16.7 and 18.6 for components A and B, respectively (Wisotzki et al. 1993). The gravitational lens nature is now firmly established, after the deflecting galaxy has been detected in the line of sight (Courbin et al. 1998; Remy et al. 1998, quoted as R98 in the following), but the lens redshift is still unknown. The strong [FORMULA] damped Ly[FORMULA] system in component A (Smette et al. 1995) may be an obvious candidate, but the observed properties do not agree well with this hypothesis (R98; Lopez et al. 1998).

Because of their brightness and the relatively large image separation of [FORMULA], photometry of the two components can be obtained under less than optimal circumstances, especially as HE 1104-1805 shows variability with considerable amplitude (Wisotzki et al. 1995). The system is therefore well-suited for systematic monitoring, with the ultimate aim to obtain the light travel time delay and to estimate the Hubble parameter. We have started with a spectrophotometric monitoring in early 1996, and including some earlier observations the time span covered is now five years. Here we present first results from this monitoring campaign, focused specifically on the issue of time delay estimation. Other aspects of the monitoring, in particular a comparison of continuum and emission line properties and the possible signatures of microlensing, will be dealt with in a future paper.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998