SIMBAD references

Trigonometric parallaxes of three planetary nebulae (A21, A29, and He 2-11) have been determined. The method of observation and reduction is briefly presented. The results for A21 and A29 are within the formal errors of the values published by USNO Flagstaff Station. The trigonometric method is discussed in comparison with other methods generally used for distance determination. We have found that statistical distances show no correlation with trigonometric distances, while other methods, which do not involve any assumption about the central stars of the nebulae, give in general fairly good correlations.