4 Conclusions

As noted in Sect. 2, the system is probably a physical one (contrary to the conclusion by Lindroos 1985). Using values as given in Table 3, V magnitudes can be corrected for extinction. In case of the CD components, (B-V)₀ for the C component was calculated taking the expected colour of the D component into account. Note that among determinations of the distance to M 20 there is some discrepancy in the data in literature. E.g., Stone (1978) gives 2.84 kpc, Avedisova & Kondratenko (1984) 1.95 kpc, Lynds et al. (1985) 1.68 kpc. With M_V values according to Schmidt-Kaler (1982) and A_V=3.1 E(B-V), an average value of the distance modulus for the A, B, C and E components is $\rm 12.2^m$, i.e., the distance equals 2.8 kpc. If A_V=1.3 according to Lynds et al. (1985), then the distance is 2.5 kpc.

No variability, photometric or spectroscopic, has been found for the star HD 164492 (ADS 10991 A). If there is any variability, then it is certainly less than announced in earlier papers, and less than our measuring errors - about $\pm$0.04 in magnitude (V) or $\pm$1 km s^-1 in radial velocity. Note also, that our investigation on HD 164492 shows the urgence to use high dispersion spectra for the radial velocity determination of OB stars.