Hδ index versus Dn(4000) diagram. Black filled circles represent the relationship derived for the MPA-JHU sample that is shown in Fig. 2. The blue line is a polynomial fit to the 1σ limit of this relationship. All objects above this line are considered E+A contaminants. Gray dots refer to the sources in the BH12-A sample with z ≤ 0.1 and S/N> 2 in the W2 and W3 bands. Red dots highlight the objects with the emission line EW< 5 Å and a FIRST central counterpart. Red dots circled in black are the nine BL Lacs candidates. The vertical dashed line indicates the limit Dn_max = 1.735.