We have compared differential zenith wet delays, estimated between the 20 m telescope and the twin telescopes at Onsala, with linear horizontal gradients from a water vapour radiometer (WVR). The east and north gradients from the WVR are projected on to the baseline between the telescopes. The formal errors of the estimated differential zenith delays are comparable to the size of the estimated values.We obtain correlation coefficients for specific 24 h experiments in the range from 0 to 0.2, and the overall correlation is 0.1. Although the correlations are low, we use simulations to verify that they are in the expected range
