We present a detailed optical spectroscopic and B , V , I , H \alpha 
photometric study of the metal-deficient cometary blue compact dwarf ( BCD ) galaxy SBS 1415+437 .
We derive an oxygen abundance 12 + log ( O/H ) = 7.61 \pm 0.01 and 7.62 \pm 0.03 ( Z = Z _ { \odot } /20 ) 12+log ( O/H ) _ { \odot } = 8.92 ( Anders & Grevesse [ ] ) . in the two brightest H ii 
regions , among the lowest in BCDs .
The helium mass fractions in these regions are Y = 0.246 \pm 0.003 and 0.243 \pm 0.010 .
Four techniques based on the equivalent widths of the hydrogen emission and absorption lines , the spectral energy distribution and the colours of the galaxy are used to put constraints on the age of the stellar population in the low-surface-brightness ( LSB ) component of the galaxy , assuming two limiting cases of star formation ( SF ) , the case of an instantaneous burst and that of a continuous SF with a constant or a variable star formation rate ( SFR ) .
The spectroscopic and photometric data for different regions of the LSB component are well reproduced by a young stellar population with an age t \leq 250 Myr , assuming a small extinction in the range A _ { V } = 0 – 0.6 mag .
Assuming no extinction , we find that the upper limit for the mass of the old stellar population , formed between 2.5 Gyr and 10 Gyr , is not greater than \sim ( 1/20 – 1 ) of that of the stellar population formed during the last \sim 250 Myr .
Depending on the region considered , this also implies that the SFR in the most recent SF period must be 20 to 1000 times greater than the SFR at ages \ga 2.5 Gyr .
We compare the photometric and spectroscopic properties of SBS 1415+437 with those of a sample of 26 low-metallicity dwarf irregular and BCD galaxies .
We show that there is a clear trend for the stellar LSB component of lower-metallicity galaxies to be bluer .
This trend can not be explained only by metallicity effects .
There must be also a change in the age of the stellar populations .
The most metal-deficient galaxies have also smaller luminosity-weighted ages .