2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/97250Results of multi-frequency VLBA polarization imaging for sources showing evidence of quasi-periodic variations in their total flux density are discussed. Possible origins of the observed behavior are proposed.3 pages, 3 figures, to appear in "Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA" J.D. Romney & M.J. Reid (eds.)Astrophysicstext