Equivalent width of Br_γ as a function of age according to SB99 models. The base model parameters/input are defined by continuous star formation (CSP), Kroupa IMF with upper mass m_u = 100   M_⊙, metallicity Z = 0.02 and Padova tracks (continuous black line). In the following models, all parameters/input of the base model are kept, except for one variable replacement: simple stellar populations (SSP) (dotted red line); Geneva tracks (dot-dashed blue line); upper mass m_u = 120   M_⊙ (dashed cyan line); upper mass m_u = 30   M_⊙ (dashed red line); metallicity Z = 0.008 (dot-dashed green line); metallicity Z = 0.05 (dot-dashed magenta line); Kroupa IMF with upper slope x = 2.0 (dotted blue line); Kroupa IMF with upper slope x = 2.6 (dotted green line).