Continuum at 230 GHz what does it mean 'The observations were made in the CO J=2–1 line and continuum at 230 GHz (1.3 mm)'?
"The observations of the continuum were also made with a bandwidth of 320 MHz. "
http://articles.adsabs.harvard.edu/pdf/2000A%26A...353L..25C

"Continuum at 230 GHz" seems like an example of oxymoron phrase?
This is the frequency that the observations were made at. These are sub-mm (the wavelength is given as 1.3mm) observations (between the far infrared and the radio parts of the spectrum) and so it is more usual to talk in terms of frequency rather than wavelength (although there are exception e.g. the 21cm line of neutral hydrogen which is in the radio at 1427 MHz). 

In the optical and infrared , we would normally say "observations were made at 550nm" but through the relationship $c=\frac{f}{\lambda}$ (where $f$ is the frequency, $\lambda$ the wavelength and $c$ is the speed of light), we could also say "these observations were made at 545077 GHz (545 THz)" although almost nobody ever would.

The "bandwidth of 320 MHz." part refers to how much of the spectrum is being sampled and digitized around the parts of interest, the CO J=2-1 line at 230.538 GHz (molecular spectra in the radio reference) and the continuum on side. The continuum is measured to get the strength of the background "underneath" the line and is then subtracted to get the true strength of the CO line.