The study of the stellar content of nearby OB associations is important for understanding the Initial Mass Function, the study of differential age effects and for a better knowledge of the ambient radiation field, which plays an important role in the interpretation of measurements of gas and dust. Unfortunately, even for the nearest OB associations membership is known very poorly. In most cases no main sequence members of spectral type later than B5 are known. Membership determination using colour-magnitude diagrams suffers from the large intrinsic distance spread. The large angular extent on the sky of most associations makes proper motion measurements difficult to compare because of problems connecting photographic plates with different plate centers. In order to remedy this situation a consortium called SPECTER has been formed at Leiden Observatory. It has been granted observing time on the HIPPARCOS satellite for measuring proper motions of about 10000 candidate members of the OB associations within 600 pc of the Sun. Candidates were selected according to spectral type (not later than F8) apparent magnitude, and location. In anticipation of the HIPPARCOS results, SPECTER will gather a variety of other data. We have nearly completed a program aimed at obtaining VBLUW-photometry (Lub and Pel 1977) of all candidate stars visible from the Southern Hemisphere. Here we discuss the preliminary results for the association Sco OB2.