We present echelle spectrophotometry of the blue compact dwarf galaxy ( BCDG ) NGC 5253 .
The data have been taken with the Very Large Telescope UVES echelle spectrograph in the 3100 to 10400 Å range .
We have measured the intensities of a large number of permitted and forbidden emission lines in four zones of the central part of the galaxy .
In particular , we detect faint C ii and O ii recombination lines .
This is the first time that these lines are unambiguously detected in a dwarf starburst galaxy .
The physical conditions of the ionized gas have been derived using a large number of different line intensity ratios .
Chemical abundances of He , N , O , Ne , S , Cl , Ar , and Fe have been determined following the standard methods .
In addition , C ^ { + + } and O ^ { + + } abundances have been derived from pure recombination lines .
These abundances are larger than those obtained from collisionally excited lines ( from 0.30 to 0.40 dex for C ^ { + + } and from 0.19 to 0.28 dex for O ^ { + + } ) .
This result is consistent with a temperature fluctuations parameter ( t ^ { 2 } ) between 0.050 and 0.072 .
We confirm previous results that indicate the presence of a localized N enrichment in certain zones of the center of the galaxy .
Moreover , our results also indicate a possible slight He overabundance in the same zones .
The enrichment pattern agrees with that expected for the pollution by the ejecta of massive stars in the Wolf-Rayet ( WR ) phase .
The amount of enriched material needed to produce the observed overabundance is consistent with the mass lost by the number of WR stars estimated in the starbursts .
Finally , we discuss the possible origin of the difference between abundances derived from recombination and collisionally excited lines ( the so-called abundance discrepancy problem ) in H ii regions , finding that a recent hypothesis based on the delayed enrichment by SNe ejecta inclusions seems not to explain the observed features .