SIMBAD references

Highly accurate radio positions of the v=1, J=2-1 SiO maser sources observed with the IRAM interferometer toward 13 late-type stars and the infrared object IRc2 in Orion, are presented. The coordinates are directly derived in the J2000 system after the baseline vectors have been oriented with respect to the extragalactic frame formed by distant quasars. We estimate the final accuracy of our `absolute' positions of SiO masers to lie in the range 0.1" to 0.2" in general. The optical and radio centers coincide within the uncertainties. However, in the complex supergiants VY CMa and VX Sgr there is a clear offset between the optical and radio positions. Spatial structure is present in our SiO data since we have observed in several stars relative position offsets larger than about 20-50 milliarcsec. The radio and optical J2000 positions are compared and discussed, and we suggest that accurate position measurements of maser sources associated with stars may valuably contribute to the alignment of the fundamental optical and extragalactic reference frames.