Procedure used to identify the two sequences of stars in the lower part of the MS of NGC 6397. Panel a: m_F150W2 versus m_F814W − m_F322W2 CMD of the stars in our sample, corrected for differential reddening and photometric zero-point variations. The two green lines (drawn by hand) are used to rectify the diagram. Panel b: rectified m_F150W2 versus Δ_{F814W − F322W2} CMD. Panels c and d: same as a and b but for the m_F150W2 versus m_F606W − m_F150W2 CMD. Panel e: Δ_{F606W − F150W2} versus Δ_{F814W − F322W2} TpCD. Panel f: Hess diagram of the TpCD. Panel g: two stellar populations identified in the TpCD (within the black envelopes), MSa (in blue) and MSb (in red). Stars outside the black envelopes are represented in grey.