Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical and radio variability of the BL Lac object AO 0235+16: a
  possible 5-6 year periodicity   New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.

31
The categories of this paper are: ao-sci
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
32
The categories of this paper are: atom-ph
30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int