1 Introduction

The 90 cm light-collector of the Leiden Observatory, situated at the Leiden Southern Station near Hartebeespoortdam, South Africa, from 1958 till 1978, has been used almost exclusively in combination with the simultaneous five-colour photometer, designed by the second author of this paper. A description of this photometer can be found in Walraven & Walraven (1960), and a discussion of the photometry based on observations with this photometer is given by Lub & Pel (1977). Many programmes carried out with this instrument have been published, but the first observational programme made with this photometer, the OB-star programme observed by the second author in 1959 and 1960, is still unpublished. The 1959 observations were reduced completely, shortly after they were made, by the second author.The 1960 observations, as far as they are available in Leiden, were reduced by the first author, who also wrote the text of this publication.