A shrink-fit coupling includes a discontinuous ring between a ceramic stub shaft of a turbine rotor and an inner diameter r2 of a metallic shaft which typically supports a compressor rotor. If the materials for the ceramic stub shaft and the discontinuous ring are pre-determined and the radius of the ceramic stub shaft is also given, then the inner radius r2 of the shaft should be chosen by the equation: r2=(CTE1−CTE2)/(CTE3−CTE2) r1.