  After almost 20 years of hunting, only about a dozen hot corinos, hot regions
enriched in interstellar complex organic molecules (iCOMs), are known. Of them,
many are binary systems with the two components showing drastically different
molecular spectra. Two obvious questions arise. Why are hot corinos so
difficult to find and why do their binary components seem chemically different?
The answer to both questions could be a high dust opacity that would hide the
molecular lines. To test this hypothesis, we observed methanol lines at
centimeter wavelengths, where dust opacity is negligible, using the Very Large
Array interferometer. We targeted the NGC 1333 IRAS 4A binary system, for which
one of the two components, 4A1, has a spectrum deprived of iCOMs lines when
observed at millimeter wavelengths, while the other component, 4A2, is very
rich in iCOMs. We found that centimeter methanol lines are similarly bright
toward 4A1 and 4A2. Their non-LTE analysis indicates gas density and
temperature ($\geq2\times10^6$ cm$^{-3}$ and 100--190 K), methanol column
density ($\sim10^{19}$ cm$^{-2}$) and extent ($\sim$35 au in radius) similar in
4A1 and 4A2, proving that both are hot corinos. Furthermore, the comparison
with previous methanol line millimeter observations allows us to estimate the
optical depth of the dust in front of 4A1 and 4A2, respectively. The obtained
values explain the absence of iCOMs line emission toward 4A1 at millimeter
wavelengths and indicate that the abundances toward 4A2 are underestimated by
$\sim$30\%. Therefore, centimeter observations are crucial for the correct
study of hot corinos, their census, and their molecular abundances.
