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We present new measurements of the magnitude of the main sequence turn offand the red giant branch bump in the luminosity function of a sample ofGalactic globular clusters with updated estimates of [Fe/H] and [$\alpha$/Fe],employing photometric data collected with the Advanced Camera for Survey onboard the HST. We compare measured and predicted values of the magnitudedifference between these two features, a rarely employed diagnostic of theinternal structure of low-mass stars at the beginning of their red giantevolution. Our analysis discloses a clear discrepancy between theory andobservations, the theoretical red giant branch bump magnitudes being too brightby on average $\sim0.2$~mag. This corroborates results from the more widelystudied magnitude difference between horizontal branch and red giant bump,avoiding the well known problems associated to the determination of thehorizontal branch level from colour magnitude diagrams, and to uncertainties inthe luminosity of horizontal branch stellar models. We briefly discuss severalpotential solutions of this discrepancy.

On the magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters