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We present the results of phase-referenced VLBA+Effelsberg observations atfive frequencies of the double-image gravitational lens JVAS B0218+357, made toestablish the precise registration of the A and B lensed image positions. Themotivation behind these observations is to investigate the anomalous variationof the image flux density ratio (A/B) with frequency - this ratio changes byalmost a factor of two over a frequency range from 1.65 GHz to 15.35 GHz. Weinvestigate whether frequency dependent image positions, combined with amagnification gradient across the image field, could give rise to the anomaly.Our observations confirm the variation of image flux ratio with frequency. Theresults from our phase-reference astrometry, taken together with the lens massmodel of Wucknitz et al. (2004), show that shifts of the image peaks andcentroids are too small to account for the observed frequency-dependent ratio.Comment: 10 pages, 8 figures, accepted by A&

VLBI phase-reference observations of the gravitational lens JVAS B0218+357