Deep broad-band BVRI photometry of a sample of 22 closely interacting isolated galaxy pairs is presented. The sample consists of pairs with a large variety of orbital geometries, but most of them are M51-type pairs: a small companion in the vicinity of a prominent two-armed spiral. This subsample will be later modelled in detail with N-body simulations. Nine of the thirteen M51-type pairs show enhanced star formation in the central regions of the companions, detected in B-V or R-I color maps, and only one pair in the nucleus of the main galaxy. This is expected if there is mass transfer from the main galaxy to the companion. Some pairs with larger separation, and with more massive companions show global instabilities manifesting as strong color gradients at least in one of the members.