Electronographic photometry of star clusters in the Magellanic Clouds - VII. The colour-magnitude diagram of NGC 2231.

Electronographic magnitudes and colours of 134 stars in the field of NGC 2231 in the Large Magellanic Cloud have been measured to V = 22.2 on electrographs taken with a Spectracon image-converter attached to the focus of the 1.5-m (60-inch) Cerro Tololo reflector. The zero point of the electronographic photometry was provided by photoelectric observations of three stars in the cluster field using the same telescope. In addition, the electronographic observations of 32 of the stars have been used to reduce the photographic observations of this cluster by Hesser et al. to the B V system. The colour-magnitude diagram of the cluster consists of an evolved main sequence, whose termination point corresponds to an age of about 1.3 × 10⁹ yr, but with a giant branch which is displaced blueward by about Δ(B-V)₀ = 0.24 from the positions of the giant branches of open clusters of comparable ages in our Galaxy. This displacement probably results from a low metal abundance, although it could also be caused by an enriched He content. On the assumption that the displacement results from the metal abundance, [Fe/H] = - 0.9. Thus, NGC 2231 is slightly older and slightly more metal-deficient than the Large Magellanic Cloud cluster NGC 2209.