Appendix A is only available in electronic form at http://www.aanda.org

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In this paper we use the expression ``line-of-sight velocity'' instead of ``radial velocity'' because the latter could be confused with the velocity radially from the star, i.e. the expansion velocity of the stellar envelope. All velocities in this paper are relative to the local standard of rest (LSR).

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... Effelsberg

The 100-m Effelsberg telescope is operated by the Max-Planck-Institut für Radioastronomie (MPIfR) at Bonn, Germany.

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... Medicina

The 32-m Medicina telescope is operated by the Istituto di Radioastronomia (IRA) at Bologna, Italy.

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... VLA

NRAO is operated by Associated Universities, Inc., under cooperative agreement with the US National Science Foundation.

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... beam

In previous works and up to 2003 April a flux density of 18.8 Jy was adopted for DR 21, following Dent (1972). In this paper, all data have been recalibrated using the value given by Ott et al. (1994).

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... System

http://www.aoc.nrao.edu/aips/

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... (AAVSO

http://www.aavso.org/

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