1. Alberdi A., Lara L., Marcaide J.M., Elosegui P., et al. (1993) Astron. Astrophys., 277, pp. L1–4.
2. Backer D. C., & Sramek R. A. (1987) AIP Conf. Proc., 155, pp. 163–5.
3. Backer D. C. (1988) AIP Conf. Proc., 174, pp. 111–6.
4. Backer, D.C., Zensus, J.A., Kellermann, K.I., Reid, M., et al. (1993) Science, 262, pp. 1414-16.
5. Backer, D. C. (1994) in Nuclei of Normal Galaxies: Lessons from the Galactic Center, ed. A. Harris & R. Genzel, [Kluwer: Dordrecht], in press.