Broadening of the charge state transition in a single-electron box

We report measurements on a sample consisting of two roughly identical single-electron transistors the islands of which are coupled capacitively. One transistor at a time is operated as an electron box. The remaining transistor is used as an electrometer to measure the charge on the box gate. While ramping up the box gate voltage transitions occur periodically between states which differ in the charge on the box island by the elementary charge e. This shows up in jumps of the electrometer current. The coupling between the box and the measuring device causes a broadening of the transition width not included in the formulae for an isolated box. This is evident in our data as well as from a thorough analysis of the system in the framework of the sequential tunneling model. The sample is studied in the superconducting as well as in the normal state.