WD effective temperature versus time as predicted by the Istrate et al. (2016) evolution track. The vertical dashed green lines delimit the age range as inferred from Fig. 8, and the red horizontal ones the corresponding WD T_eff range. The horizontal blue line indicates the effective temperature measured by Bassa et al. (2006), while the vertical one the corresponding cooling age. Dashed blue lines delimit 1σ uncertainty ranges.