Vanishing Effective Mass of the Neutrinoless Double Beta Decay?

We stress the point that massive neutrinos may be Majorana particles even if the effective mass of the neutrinoless double beta decay 〈m〉ee vanishes. We show that current neutrino oscillation data do allow 〈m〉ee = 0 to hold, if the Majorana CP-violating phases lie in two specific regions. Strong constraints on the neutrino mass spectrum can then be obtained. A possible texture of the neutrino mass matrix is also illustrated under the 〈m〉ee = 0 condition.