2 Astrometric measurements

For 87% of the stars listed in the catalogue, the right ascension and declination were determined from 15 direct B plates (IIIa-J emulsion; scale: 50 arcsec/mm) obtained with the ESO 40 cm GPO, at La Silla. The measurements were performed with the OPTRONICS machine at ESO-Garching. The data were reduced using the POS1 astrometric programme written by West (1981), with the implementations added by O. Hainaut. An average of 30 reference stars per plate, selected from the PPM catalogue (Bastian et al. 1991), was used. Morgan (1999) provided to us the 2000.0 coordinates of the new ${\rm W}-{\rm R}$ star (MG 9) he recently found in the LMC.

For 9% of the stars, located in crowded regions and therefore difficult to measure on the photographic plates, the positions were determined on CCD images (HeII 4686 interference filter; scale: 0.26 arcsec/pixel) secured with the direct camera attached to the 2.2 m telescope, at La Silla. The equatorial coordinates of these ${\rm W}-{\rm R}$ stars were calculated by linear interpolation using as secondary standards nearby stars measured on the astrographic plates. The central stellar positions were established using a two-dimensional Gaussian fit. For the remaining 4% of the stars - six members of the R 136 cluster at the heart of the 30 Doradus nebula - we adopted the coordinates published by Malumuth & Heap (1994).

For the stars appearing on different plates or CCD frames, the astrometric measurements were repeated. This resulted in 30% and 10% of the stars having their coordinates determined independently two times and three times respectively. An estimation of the internal error is therefore possible. The obtained accuracy is $\pm 0.045$s in right ascension, and $\pm 0.20$ arcsec in declination. For those stars in 30 Doradus where the comparison is possible, our coordinates are in excellent agreement with the ones given by Parker (1993) and Malumuth & Heap (1994).