This paper analyzes simultaneous self-potential and gas temperature variations recorded at Merapi volcano in spring 2001, the dry season shortly after the volcanic crisis 2001. Temporal variations of fumarole gas temperature show characteristic quasi periodic signals at scales 1-8 hours and amplitudes up to ten degrees. We propose a simple graphical technique combining a wavelet scalogram and a cross-correlation analysis to demonstrate that the variations of gas temperature are systematically preceded by self-potential variations at the same scales. The influence of meteorological variations on these correlated signals can be ruled out. Rather, we suggest them to be related to the magma degassing in the upper conduits of the volcano. We discuss a semi-qualitative model to explain this correlation and the observed phase shift of about two hours.