Rapid polarization variations at 20 cm in 0917+624

The intraday variations (IDV hereafter) of the total and the polarized flux density observed at 20 cm in 0917+624 tin May 1989) are analysed. It is sh own that the IDV can be interpreted in terms of refractive interstellar sci ntillation. The observed variations are dominated by one scintillating comp onent with a timescale tau approximate to 0.6 days and a scintillation inde x m approximate to 0.04. In addition, one more scintillating component with a shorter timescale and a smaller scintillation index is needed to improve the fit to the observed Q- and U-light curves. The relationship between th e scintillating components and the VLBI components are also discussed. It i s shown that Doppler beaming of the 20 cm scintillating components with Lor entz factor greater than or similar to 7 may be needed to reduce their intr insic brightness temperatures significantly below the inverse-Compton limit .