Radio monitoring of a sample of X- and gamma-ray loud blazars

In this paper we present the results of a 4-year (1996-1999) radio flux den sity monitoring program for a sample of X- and gamma -ray loud blazars. Our program started in January 1996 and was carried out on a monthly basis at the frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located at Medi cina (Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz data collected at Medicina from January 1996 to June 1997 will also be presented. The sample of selected sources comprises most radio-loud blazars with delta greater th an or equal to -10 degrees characterised by emission in the X- and gamma -r ay regimes, and target sources for the BeppoSAX X-ray mission. All sources in the sample, except J1653+397 (MKN501), are variable during the four year s of our monitoring program. We classified the type of variability in each source by means of a structure-function analysis. We also computed the spec tral index alpha (8:4)(5) for all epochs with nearly simultaneous observati ons at these two frequencies, and found that alpha (8:4)(5) starts flatteni ng at the very beginning of a radio flare, or flux density increase.