1 Introduction

The present paper is a continuation of our study of M-type asteroids (Belskaya & Lagerkvist 1996). The purpose is to enlarge the data set of rotation periods of asteroids of taxonomic type M. Previous studies of the spin rates of M-type asteroids have shown, with varying statistical significance, that on average M-type asteroids have faster spin than asteroids of other taxonomic types (Harris & Burns 1979; Dermott & Murray 1982; Lagerkvist 1983; Lagerkvist et al. 1985; Binzel et al. 1989; Belskaya & Lagerkvist 1996).

There are two main problems in analysing the distribution of spin rates for M-type objects. One is the small number of M-type asteroids with known rotation periods. In this paper we present observations of nine M-type asteroids, resulting in new determinations of spin rates for six of these. This gives an increase of more than 15$\%$ of the known spin rates of asteroids of taxonomic type M. In order to increase the known population of M-type asteroids we have determined B-V colour indices for 17 asteroids with moderate albedos. The second problem is the inconsistent classification of some asteroids between different taxonomies. Below we discuss the reliability of the classification for some peculiar objects.