= First massive level of the open RNS string
{title2=$\alpha'M^2=1,\quad128_B+128_F$}
After the <GSO projection>, the first massive open-string level has NS oscillator number $3/2$ and Ramond oscillator number $1$. Eight transverse directions give $8+64+\binom83=128$ NS states and $8\cdot8+8\cdot8=128$ R states. They form the massive Spin(9) representations $44+84$ and a gamma-traceless vector-spinor of dimension $9\cdot16-16=128$. The identical chiral oscillator count in a <closed string> has the different mass normalization $\alpha'M^2=4$.
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