...quasar

Based on observations collected at the European Southern Observatory (La Silla, Chile) and with the Nordic Optical Telescope (La Palma, Spain).

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...quasar^,

Table 1. Logbook for the ESO and NOT observations together with photometric results for the Cloverleaf quasar. This long table can be accessed on the WWW at the URL address: http://vela.astro.ulg.ac.be/grav_lens/glp_homepage.html

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...Surdej^,

Research Director, Belgian Fund for Scientific Research (FNRS).

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...Gosset^,

Research Associate, Belgian Fund for Scientific Research (FNRS).

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...Magain

Senior Research Associate, Belgian Fund for Scientific Research (FNRS).

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