Solving eq. 8.5 by using the experimentally determined shift of the optical gap E, an estimate for the change in reduced mass can be obtained. As shown in table , in the case of CsPbBr3 the decrease in reduced mass from 0.072 to 0.045 can be achieved by halving for example the effective hole mass to 0.07 me. Similarly, for CsPbBr2Cl an even stronger reduction can take place, while the electron mass could also be concurrently reduced.