Context :

Aims : We derive the structure , distribution of MS and PMS stars and dynamical state of the young open cluster NGC 4755 .
We explore the possibility that , at the cluster age , some MS and PMS stars still present infrared excesses related to dust envelopes and proto-planetary discs .

Methods : J , H Â and K _ { s } Â 2MASS photometry is used to build CMD and colour-colour diagrams , radial density profiles , luminosity and mass functions .
Field-star decontamination is applied to uncover the cluster ’ s intrinsic CMD morphology and detect candidate PMS stars .
Proper motions from UCAC2 are used to determine cluster membership .

Results : The radial density profile follows King ’ s law with a core radius \mbox { $ R _ { core } $ } = 0.7 \pm 0.1 pc and a limiting radius \mbox { $ R _ { lim } $ } = 6.9 \pm 0.1 pc .
The cluster age derived from Padova isochrones is 14 \pm 2 Myr .
Field-star decontamination reveals a low-MS limit at \approx 1.4 \mbox { $ M _ { \odot } $ } .
The core MF ( \chi = 0.94 \pm 0.16 ) is flatter than the halo ’ s ( \chi = 1.58 \pm 0.11 ) .
NGC 4755 contains \sim 285 candidate PMS stars of age \sim 1 - 15 Myr , and a few evolved stars .
The mass locked up in PMS , MS and evolved stars amounts to \sim 1150 \mbox { $ M _ { \odot } $ } .
Proper motions show that K _ { s } -excess MS and PMS stars are cluster members . K _ { s } -excess fractions in PMS and MS stars are 5.4 \pm 2.1 \% and 3.9 \pm 1.5 \% respectively , consistent with the cluster age .
The core is deficient in PMS stars , as compared with MS ones .
NGC 4755 hosts binaries in the halo but they are scarce in the core .

Conclusions : Compared to open clusters in different dynamical states studied with similar methods , NGC 4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass .
On the other hand , the flatter core MF probably originates from primordial processes related to parent molecular cloud fragmentation and mass segregation over \sim 14 Myr .
Star formation in NGC 4755 began \approx 14 Myr ago and proceeded for about the same length of time .
Detection of K _ { s } -excess emission in member MS stars suggests that some circumstellar dust discs survived for \sim 10 ^ { 7 } yr , occurring both in some MS and PMS stars for the age and spread observed in NGC 4755 .