The coupling of the velocity and temperature dynamics is studied in a non-isothermal mixing layer air flow using a new variable temperature hot wire anemometer, designed and implemented for the simultaneous measurement of velocity and temperature at the same point with a single wire. Temperature probability density functions are found to be non-marching in the development region and marching in the self-similarity region. The synchronism of the velocity-temperature measurements makes possible a fine analysis of the velocity and temperature fluctuations interactions in the turbulent mixing process: correlations, conditional probability density functions and quadrant analysis pointed out the dominance of saddle-point ejections in the eddy heat flux.